No, Lonnie cannot just fire Jerry. The Americans with Disabilities Act (ADA) requires employers to reasonably accommodate applicants and employees with disabilities to enable them to enjoy equal employment opportunities unless doing so would be an undue hardship (i.e., a significant difficulty or expense). Accommodations vary depending on the needs of the individual with a disability.
The employer should work with Jerry to determine whether he can be reassigned to a vacant position for which he is qualified. The vacant position must be equivalent in terms of pay and status to the original job, or as close as possible if no equivalent position exists. The position need not be a promotion, although the employee should be able to compete for any promotion for which he is eligible.
Employers should ask an employee with an intellectual disability, or the person acting on his or her behalf, what the employee needs to help him or her do the job. Also, extensive public and private resources are available to help employers identify reasonable accommodations.
Source: EEOC Guidance: "Revised Questions and Answers about Persons with Intellectual Disabilities in the Workplace and the Americans with Disabilities Act," https://www.eeoc.gov/laws/types/intellectual_disabilities.cfm, reported in Employment Practices Guide, ¶5374.